Effect of pH on Stability of Oil-in-water Emulsions Stabilized by Pectin-Zein Complexes

The purpose of this study was to investigate the influence of pH on properties and stability of oil-in-water emulsions using pectin as an emulsifier and zein as an auxiliary emulsifier. Primary emulsions were prepared by mixing rice bran oil with anionic polysaccharide, pectin, at pH 4 or 7. Zein solution was then added in order to prepare secondary emulsions. Final concentration of secondary emulsions was 20% (w/w) rice bran oil, 2% (w/w) pectin and 0.5% (w/w) zein. The results demonstrated that emulsions prepared at pH 4 were smaller in size, higher viscosity, and more stable after stability test than those prepared at pH 7. Light microscopic images showed that, at pH 7, emulsion droplets were aggregated. At pH 7, both pectin and zein represented as negative charge and no complex was formed, leading to the less stable emulsions. Below the isoelectric point (pH approximate to 5.5) of zein, i.e. at pH 4, zein molecules represented as positive charge and pectin represented as negative charge, resulting in a light cross-linking polymer network. It is possible that a low concentration of zein helps to stabilize the emulsions by improving the rigidity of coated layer on oil droplets, thus reducing the depletion flocculation to occur. The stabilizing effect in presence of zein, at pH 4, is attributed to a pectin-zein complex formation at the interface.